Nutrient Control In Waste Water Facilities
On line monitoring and control of nutrient removal or addition in biological processes, hasn't, until recently, found widespread application.The reasons for this were lack of reliable "In situ" instrumentation together with process variants and physical structures, which did not lend themselves easily to dynamic control. The advent of batch and semi batch biological processes, and aeration control developments for plug flow units, together with recent instrument advances, has changed this.
Plant loading varies with time; hence it is not possible to make optimum provision at all times merely by physical design. If we are to move forward we must design in variable treatment provision controlled by real time measurement of the important variable parameters. This may be done in many ways, but all have two inescapable pre requisites:
(a) An initial plant design which builds in the capability for variation and
(b) Availability of reliable, affordable instrumentation.
Most waste water treatment plants of significant size utilise some variant of the activated sludge process. They tend to fall into 2 categories: Nutrient rich, requiring removal of some or all of NH3, NO3 and PO4, and Nutrient deficient, requiring addition of one or more nutrients. Municipal works are generally of the former and Industrial often of the latter. In both cases however it is necessary to measure continuously the concentration of the nutrients and to control the removal or addition mechanisms accordingly, if one is to minimise running costs and maximise quality assurance.
Case study 1 : SBR Control for treatment of digested sludge centrate liquors.
Liquor generated from anaerobically digested sludge centrate was partially treated to reduce the Ammonia concentration to acceptable levels, before going onto further aerobic treatment in admixture with settled sewage. An SBR type system was used for this purpose. The fill and draw cycle was controlled on a timer, and aeration control based on a timer-controlled variable D.O. set point.
Improvement in efficiency could be obtained by measuring NH3-N in situ and controlling the aeration period to achieve the desired level of removal.
Transferring a sample from the tank and pretreating it prior to analysis was
found to be impossible due to the very high content of fine shredded plastic
material, which caused severe blockage problems. Consequently a prerequisite
for improving control was the implementation of an insitu device for Ammonia
measurement. The Stip process buoy supported on a floating collar was
used for this purpose.
Schematic of an SBR Centrate Treatment Plant
Under normal operating conditions it was found that the concentrations of
Ammonia in the reactor tank were far less than expected or desired. This
infers either that treatment is excessive or the loading on the plant
could be increased considerably. In either case the efficiency could be
improved, giving improved operational costs . The monitoring system has
been proved and the feasibility study completed, the final implementation
is now to be completed. The principle of using the Ammonia measurement
to control the aerobic time of treatment is also applicable to plug flow
plants, and it has been estimated that on a medium to large works cost
savings of approximately £15K/yr may be achievable.
Case study 2: Insitu phosphate measurement and removal
Isco -Stip have introduced a novel in-situ phosphate monitor, which now allows phosphate measurement within the aeration tank. Hence it is now an easy matter to continuously monitor phosphate levels in biological processes and to utilise the output to automatically control either removal or addition of phosphate.
A German wastewater facility, where phosphate removal is achieved through the addition of Aluminium Sulphate, operated initially without monitoring the phosphate levels for control, and then implemented a monitoring and control scheme. This reduced the daily consumption of Chemical from 845kg/d to 655 kg/d, with an annual saving of some 11000 Euros.
The recent development of in-situ nutrient measurement technology and the advances
in process control and operational modes, now means that it is a viable option
to utilise continuous measurement and control, for the improved efficiency of
nutrient removal or addition in wastewater processes.